The rat's circadian rhythms of wheel-running and of internal temperature will be studied with a view to clarifying: 1. The basic nature of the hypothesized internal clock. (a) In progress is an examination of the phase-resetting mechanism as it operates under atypical illumination regimens, which may narrow the divergencies between the traditionally opposed endogenous and exogenous points of view. (b) In progress is an investigation of the changes in activity pattern as a function of aging. (c) To be extended is an earlier demonstration suggesting that exposure to a non-circadian illumination schedule from before birth can effect a persisting distortion of the free-running period. 2. The phenomenon of internal desynchronization, particularly as it may induce pathology. (a) The effect of repeated desynchronization produced by reversal of the light/dark schedule, will be assayed in terms of early growth, susceptibility to audiogenic seizure and to stress-induced ulceration, sexual maturation, and longevity. (b) Age and other factors influencing frequency of spontaneous desynchronization, resistance to induced desynchronization, and rate of recovery, will be examined. 3. The pattern of adaptive response to extremes of ambient temperature. (a) The prediction will be tested that increases in running in response to increases of ambient temperature are most likely to occur during the hours of lowest internal temperature.